Most Americans have never heard of Anwar al Awlaki, but the radical Muslim cleric who may have inspired a young Nigerian man to try to blow up a plane on Christmas Day has been linked to the alleged perpetrators of the deadliest terror attacks on U.S. soil this decade, from 9/11 to the massacre at Fort Hood.

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Awlaki twice made headlines last week. Along with several other al Qaeda operatives, the preacher was the target of a U.S. airstrike in Yemen Dec. 24. He was mentioned in articles around the world again the following day, when one acolyte, suspected terrorist Umar Farouk Abdulmutallab failed to detonate an explosive device while aboard a Northwest Airlines jet bound for Detroit on Christmas.

Awlaki apparently is still alive after the Yemen attack, and has reportedly made calls to Yemini journalists in the last few days. This would not be the first time U.S.-born imam slipped through the hands of American justice.

With much of the senior al Qaeda leadership believed to be in hiding in Pakistan, Awlaki has been able in recent years to fill a niche for those looking to hear and read extremist interpretations of Islam, said Kenneth Katzman, a Middle East and terrorism specialist at the Congressional Research Council.

"Some of the benefit of Awlaki was that he wasn't being chased. For a long time he was not under severe pressure, when others were," said Katzman. "He only came to prominence after the Fort Hood shootings. He was not being heavily pursued, and that afforded him a certain amount of liberty, to preach and let people contact him."

After several years of preaching an extreme version of Islam across mosques in the United States, the 36-year-old imam left the U.S. in 2002 following a decision that forced federal authorities to rescind a warrant issued for his arrest related to alleged passport fraud.

Once in Yemen, Awlaki, a U.S. citizen born in New Mexico, hooked up with the local branch of international terror syndicate al Qaeda, according to U.S. authorities. Despite moving to the Middle East, through his Web site and audio recordings he preached a message of violence and hate that became popular with a new generation of Muslims raised outside the Arabian Peninsula.

Al Qaeda in the Arabian Peninsula, the Yemen-based branch with which Awlaki is affiliated, took credit Monday for Abdulmutallab's botched bombing on Christmas Day.

Awlaki has "no known direct links with al Qaeda Central, but [he has them] with al Qaeda in Yemen," said Bruce Hoffman, a professor and terrorism expert at Georgetown University, in an e-mail.

"[He] is seen as effective in communicating with and radicalizing Western jihadis and wannabe jihadis. … He is a very effective and charismatic communicator," Hoffman said.

Contacts with Terrorists

Among those drawn to Awlaki's message of jihad were men convicted of plotting terror attacks in Toronto and Fort Dix, N.J. Both the British-educated Abdulmutallab and Maj. Nidal Hassan, a U.S. Army psychiatrist accused of gunning down 13 people in November at Fort Hood, were said to be influenced by the preacher.

Awlaki also consulted with two of the 9/11 hijackers, Nawaf al Hamzi and Khalid al Mihdhar, in San Francisco. Meetings, a congressional inquiry investigating the attacks said, "may not have been coincidental."

Beginning in 2008 Awlaki began an e-mail correspondence with Nidal Hassan in which the Army doctor asked for and received religious justifications for murder. The two men exchanged some 20 e-mails, according to government investigators.

In one such e-mail, Hassan wrote Awlaki: "I can't wait to join you" in the afterlife.

Following the Fort Hood attacks Awlaki called Hassan a "hero" on his Web site.

"The only way a Muslim could Islamically justify serving as a soldier in the U.S. Army," he wrote, "is if his intention is to follow the footsteps of men like Nidal."

On Monday, the government of Yemen confirmed that failed bomber Abdulmutallab had visited that county in early 2009, raising suspicions that the would-be terrorist met with his spiritual leader.

"It appears that just like with Major Hasan, Awlaki played a role in this," Rep. Pete Hoekstra, R-Mich., ranking minority member of the House Intelligence Committee, told ABC News' "The Blotter."

Paths to Radicalization

"All roads point back to Yemen, they point back to Awlaki, I think it is a pretty deadly combination," Hoekstra said.

The congressman told "The Blotter" that he would investigate how Abdulmutallab was radicalized, and whether he went to Yemen on his own initiative to meet members of al Qaeda.

Many of the young men who find Awlaki's message of murder and martyrdom inspiring are disaffected youths, who often feel alienated not just in the countries in which they live but within their own families, said Anthony Cordesman, an expert at the Center for Strategic and International Studies.

"These are young men looking for something and who can be easily persuaded. It is difficult to know how many are found through the appeal of the message and a charismatic leader, versus structured recruiting," Cordesman said.

"As a native English speaker born in the U.S., it makes it easier to reach out to Americans, and the host of other Muslims alienated by living in the West," he said.

Awlaki was born in New Mexico in 1971 while his father went to college. His family moved back to Yemen for a time, but Awlaki returned in 1991 to study engineering at Colorado State University.

Following his graduation, he became an imam, leading mosques in Fort Collins, Colo., San Diego and later outside Washington, D.C.

According to a Roanoke, Va., newspaper, Awlaki presided over the funeral of Nidal Hassan's mother while in Virginia.

After 9/11, authorities looked for a reason to legally detain and question the fiery preacher who was known to have met with two of the hijackers.

Awlaki was arrested twice in the 1990s for soliciting prostitutes, and federal authorities in 2002 hoped to catch and arrest him for violations of the Mann Act, a law that prohibits the transportation of people across state lines for illegal purposes.

Though born in New Mexico, Awlaki claimed in 2002 that he was born in Yemen when applying for a visa to attend college in Colorado -- an alleged incidence of passport fraud.

A warrant was issued for his arrest, but one day before Awlaki was detained in an airport on his way to Yemen, investigators learned that an assistant district attorney had rescinded the warrant.

U.S. Attorney David Gaouette defended the decision to rescind a 2002 felony arrest warrant for Awlaki, telling ABC News' "The Blotter" that his office determined there was insufficient evidence to pursue the case.

As reported by ABC News, the warrant for Awlaki, an al Qaeda recruiter and self-described "confidant" of alleged Fort Hood shooter Nidal Hasan, was rescinded a day before Awlaki was intercepted as a terror suspect at JFK airport in New York in October 2002. Authorities had to release him, and he soon left the country for Yemen.

The warrant would have given terror investigators the right to hold Awlaki, who was on a terror watch list, on charges of felony passport fraud. "It was a determination of our office that we couldn't prove the case beyond a reasonable doubt and we asked the court to withdraw the complaint," Gaouette, U.S. Attorney for the District of Colorado, told ABC News. Gaouette, who in 2002 was the assistant U.S. attorney who supervised the Awlaki case, said he took responsibility for rescinding the warrant.

Gaouette also said that Awlaki's terrorism-related background had no bearing on the decision to drop the case. He said he could not continue with a case just "because someone has a bad reputation."